The staff of the Clinical Studies Section in Boston, Massachusetts has sifted a large volume of clinical data to identify patients for etiologic studies. These unusual patients are usually found through attendance at clinical conferences and rounds, and through special referrals. With informed consent, epidemiologic studies are made of the environmental and genetic background and laboratory studies are conducted to clarify biologic mechanisms of carcinogenesis. At present, several family aggregates of cancer are under study, e.g., Turcot's syndrome in two families; Gardner's syndrome variant; breast cancer-Hodgkin's disease aggregation; autosomal dominant renal cell carcinoma associated with a 3:8 chromosome translocation. In addition, a family suffering from mesothelioma following asbestos exposure at a shipyard has prompted two studies of mesothelioma and lung cancers in two New England shipbuilding communities. Also, a computerized Registry of Survivors of Childhood Cancer has been established through collaboration of the Clinical Studies Section with the Sidney Farber Cancer Institute. The 600 registrants represent approximately 2% of all survivors of U.S. childhood cancer in the last two decades. Patients in the series are being studied for second malignancies, cancer in close relatives, and genetic effects in progeny.